In recent years, demands of a mobile communication terminal as a wireless communication means, such as a car telephone, a mobile phone in a train and on the street, has been explosively increased worldwide. To make the mobile communication possible, not only the network system itself but also terminals for interfacing with users directly are required to have a small size, light weight, low power consumption and high performance.
Actually small size parts used in the terminals are the ones contributing to the size reduction of the mobile terminals. Particularly, SAW devices are currently used as band-pass filters, resonators, delay lines, and convolvers, in a wide range of RF and IF applications, for example, wireless, cellular communication, and cable TV.
In general, the SAW is material wave generated by motions of particles in presence of thermal, mechanical and electrical forces from the outside, and exists only in solid or liquid.
Basically, waves can be divided into three kinds: longitudinal waves, in which the direction of wave propagation is parallel to the particle displacement, transversal waves, in which the direction of wave propagation is perpendicular to the particle displacement (oscillation), and shear waves, which are obtained by adding horizontal vectors and vertical vectors.
The most effective and general method for generating or detecting the SAW device out of piezoelectric substrate is fabricating IDT (InterDigital Transducer) structures. The IDT aligns metal electrodes in parallel on the piezoelectric substrate, and the pattern at this time is similar to the time impulse pattern.
Mostly IDT of each electrode is fabricated through. aluminum deposition, and sometimes aluminum alloy is also used to increase voltage-resistant property. Also, Ti or specific alloy is used to improve contact property of aluminum. In general, the width of aluminum used ranges from 0.5 μm to 15 μm.
FIG. 1 is a schematic perspective view of a SAW device of the related art.
As shown in FIG. 1, when an alternating signal voltage is applied to an input IDT 101, electric field is generated between other neighboring electrodes with different polarity and piezoelectric effect is created on the surface of a substrate 104. As a result, the surface of the substrate 104 is transformed and SAW is propagated to both directions of the input IDT 101.
FIG. 2 is a schematic diagram illustrating transformation brought on the inside of the piezoelectric substrate due to SAW according to the related art.
As depicted in the drawing, when SAW is propagated, the substrate is transformed and the SAW is transferred in the form of mechanical energy.
Then an output IDT 102 on the opposite side detects the energy when SAW is propagated, using inverse piezoelectric effect from where the energy is formed to each electrode.
On the other hand, to block unnecessary reflective waves, both ends of the surface of the substrate 104 can be coated with an acoustic absorbent 103.
As for the acoustic absorbent, rubber, silicon gel, photosensitive film, or polyamide can be used, and its coating shape is also diverse.
Therefore, characteristics of the SAW device are determined by input/output IDT patterns for converting electrical signals to mechanical energies or mechanical energies to electrical signals, and by how the pattern sizes are adjusted.
Usually the SAW device with the above construction is used as the band-pass filter. This application turned out to be very beneficial for reducing weight and size, increasing reliability and reducing power consumption. Typically used SAW filters are transversal SAW filters having two IDTs disposed on the piezoelectric substrate at a predetermined distance, resonator filters mounted with a resonator on the piezoelectric substrate, and combination filters.
To develop the SAW filter, technologies involved in electrode designing, patterning, SMD packaging, measurement of RF characteristics, and circuit designing for used in impedance matching should be organically correlated to each other and systemized.
In general, quartz, lithium niobate (LiNbO3), ST-quartz and lithium tantalate (LiTaO3) are used to fabricate a SAW single crystal substrate used in patterning for SAW propagation.
Because the SAW filter is heavily influenced by properties of those piezoelectric single crystal substrates that generate and propagate surface acoustic waves, it is very important to set up specific orientations for different properties and to cut the substrates accordingly.
The properties to be considered are SAW velocity, SAW coupling coefficient, power flow angle (pfa), diffraction or beam spreading coefficient, Y (gamma), or temperature coefficient of delay. Depending on these properties, the SAW device may or may not obtain radio frequency.
Especially the first order temperature coefficient of delay (tcd) among other properties is very sensitive to frequency change, and thus the usefulness of the SAW device as a temperature sensor is very high.
FIG. 3 is a schematic diagram of a SAW device of the related art being used as a temperature sensor.
As illustrated in FIG. 3, there are input IDT and output IDT on a single crystal substrate. When a voltage is applied to the input IDT, electric signals are converted to mechanical energies and SAW is propagated along the single crystal substrate. Here, if temperature is changed, frequency of the SAW to be propagated is changed also. Then the SAW with a changed frequency, now being in the form of mechanical energy, is converted back to electrical signals at the output IDT and outputted therefrom.
The frequencies of output signals are amplified through an amplifier, and the amplified signals are transmitted wirelessly. When the transmitted signals are received, the frequencies of the signals are measured and temperatures corresponding to frequencies are detected (or sensed).
Following result is obtained by applying quartz Euler angles of the single crystal substrate, that is, φ=0°, θ=15.7° and ψ=0°, to the SAW device.
Vs (km/s)=3.948582, Vo (km/s)=3.95077, K2 (%)=0.1108, pfa (deg)=0, tcd (ppm/C)=0.25181, tcd2 (1e−9/C^2)=−1.8167, loss_s (dB/λ)=0.0003059, and loss_o (dB/λ)=0.0003297.
Where, Vs and Vo are phase velocities for shorted and opened (free) surface respectively, K2 is coupling coefficient, pfa is power flow angle, tcd and tcd2 are first and second order temperature coefficient of delay respectively, and loss_s and loss_o are propagation loss for shortened and opened surface respectively.
On the other hand, following result is obtained by applying lithium tantalate (LiTaO3) Euler angles of the single crystal substrate, that is, φ=10°, θ=23.6°, and ψ=78.8°, to the SAW device.
Vs (km/s)=2.969688, Vo (km/s)=2.972704, K2 (%)=0.2029, pfa (deg)=0.03048, tcd (ppm/C)=−0.06127, and tcd2 (1e-9/C^2)=−3.496.
To be short, when the above quartz substrate Euler angles or lithium tantalate substrate Euler angles are applied to the temperature sensor, although the properties do not go through great changes, one cannot obtain an optimal temperature coefficient that is needed for the SAW device to be used as the temperature sensor.
Therefore, it is very important to set specific orientations according to different material properties of the single crystal substrate, namely quartz, langasite or lithium tantalate substrate, when it is applied to the SAW device, and to cut the substrate accordingly. Also, if the SAW device has lower values than demanded, performances of the SAW device are consequently deteriorated.